Continuing studies will be made of the biochemical mechanisms by which snake venom proteinases retain or regain activity in the presence of molar excesses of plasma proteinase inhibitors (PPI), the major host defense against exogenous proteolytic activity. The active modified C1 esterase inhibitor (C1INH) formed by Crotalus atrox Alpha protease and by C. adamanteus Proteinase H, the hemorrhagic proteinase from this venon will be studied and reaction products isolated and characterized to determine the regions of the C1INH molecule which are susceptible to limited proteolysis. The modified inhibitor will be isolated in homogenous condition, and the stoichiometry of complex formation between modified C1INH and plasmin, kallikrein and C1 esterase determined and compared to that of intact C1INH. Stability of the complexes will be studied by electrophoresis and assays for reappearance of proteolytic activity. Susceptibility of modified C1INH to leucocyte elastase will be determined and compared with intact C1INH to establish whether modification by exogenous venom proteinases enhances C1INH inactivation by endogenous proteinases. Alpha1 antichymotrypsin (Alpha1X) was the only PPI studied which was inactivated by Elapid venoms, and the most active venom, Dendroaspis angusticeps, will be fractionated and the proteinase which inactivates Alpha1 X purified and characterized. Possible effects on other PPI will be examined to determine whether this proteinase is indeed specific for Alpha1X. Studies of the inactivation reaction, including isolation of products, will be made and compared to Alpha1X inactivation by C. atrox Alpha protease. High molecular weight venom proteinases (C. adamanteus Proteinases H and B, and C. atrox Gamma protease) are now available in nearly homogeneous form. The pure enzymes will be incubated with Alpha2-macroglobulin (Alpha2M) at varying ratios, residual proteinase and inhibitor activity measured, and the reaction monitored electrophoretically to determine the extent of interaction with Alpha2M and the stability of any complexes formed, and to show how venom proteinases retain activity in the presence of Alpha2M. These studies should increase understanding of the role of proteolysis in snakebite pathogenesis by demonstrating that enzymatc inactivation of PPI is a mechanism by which trace amounts of venom proteinases can effectively disrupt the PPI system. Enzymatic inactivation of PPI by specific venom proteinases could also prove useful in animal model systems for determining the physiological roles of PPI and for studying diseases, e.g., emphysema, involving deficiencies of a specific PPI.